This invention relates to integrated optical spectrum analyzers and more particularly to a method of improving the signal-to-noise ratio of such devices.
Integrated optical spectrum analyzers (IOSA) are under development as set forth in the article, "An Integrated Optical RF Spectrum Analyzer", by M. C. Hamilton et. al., Optical Engr. 16, 475-478, Sept./Oct. 1977. Briefly the IOSA is a planar waveguide into which a laser is end-fire-coupled, collimated, Bragg-diffracted by a surface acoustic wave, and focused on a detector array. The detector array must be external to the optical waveguide system (Ti:LiNbO.sub.3) which is of interest for this disclosure. An important consideration in such a device is the usable signal-to-noise ratio or dynamic range. Dynamic range is limited by any optical radiation which reaches the photosensitive detectors other than the desired optical signal. Such optical background noise typically arises from scattered light which is lost from the input laser beam and reflections of both the primary input laser beam and secondary scattered light from it. In some circumstances radiation from the guided optical laser beam may also give rise to a background signal. Typically scattering is due to surface imperfections and edge imperfections on input and output coupling. Additional background sources are both laser light and spontaneous emission from the laser which is coupled into the substrate.
Of the two materials systems currently of interest for IOSA applications, silicon (Si) and lithium niobate (LiNbO.sub.3), the problem is not so severe for the Si system because the Si substrate is highly absorbing for the optical radiation at the wavelength of interest (0.83 .mu.m). Then all light in the substrate is absorbed and no reflections or scattering from the bottom surface of the substrate can reach the photodiodes. LiNbO.sub.3, however, is transparent at a wavelength of 0.83 .mu.m and LiNbO.sub.3 substrates will not attenuate any light in the substrate. Reflections are substantial in LiNbO.sub.3 because the high index of refraction (2.2) of the material causes a normal-incidence reflectivity of approximately 15%. Low-angle reflections can be much larger, approaching 100%. Scattered or reflected light from the bottom surface of the substrate is a severe limitation on device dynamic range.